The present invention relates, in general, to resistance heaters and methods for their formation, and more particularly, relates to tubular resistance heaters suitable for heating fluids.
Resistance heaters are in widespread use and are constructed in a number of different physical geometries including heater rods, plates and tubes. Moreover, such heaters have been formed using various electrical resistance heating elements, including resistance wires, silicone blankets, thick film in-line paths and thin film areas.
Tubular heaters have been found to be particularly effective in heating fluids, namely, gases and liquids, by flowing the fluid down the inside or over the outside (with a containment structure) of the tubular heater. Resistance wires, blankets and thick film paths have all been previously employed to form tubular resistance heaters, but each of these technologies has been found to have attendant disadvantages.
An example of the use of thick film technology to form a tubular resistance heater is set forth in the advertising flier of Watlow Industries of Atlanta, Ga. entitled xe2x80x9cThick Film In-Line Heaters on Quartz Provide Long Life and Efficient Heat Transfer.xe2x80x9d Thick film tubular resistance heaters are efficient and they can achieve high watt densities. Thick films, however, are not molecularly bonded to the supporting substrate so they can experience durability problems. Since they employ an xe2x80x9cin-linexe2x80x9d film path, as the diameter of the tube decreases, the thick film paths become more and more crowded, making them poor candidates for small diameter tubular heaters, for example, heaters for medical catheters.
Similar problems can be encountered when tubular resistance heaters are formed by adhering resistance heater wires to a substrate or when encircling a tubular substrate with a silicone blanket.
Accordingly, it is an object of the present invention to provide a tubular resistance heater, and method for its formation, which has the advantages of efficient heat transfer to fluids, but which also has improved durability and can be formed in very small diameters.
Another object of the present invention is to provide a tubular resistance heater which is easy to construct, can be employed with a variety of substrates and tube sizes, is highly efficient in transferring heat, is compact, and can be constructed for use in many heating applications.
The tubular resistance heater and method of the present invention have other objects and features of advantage which will be apparent from, and are set forth in more detail in, the accompanying drawing and following description of the Best Mode of Carrying Out the Invention.
The tubular resistance heater of the present invention comprises, briefly, a tubular substrate having an electrically non-conductive surface; a thin film electrical conductor deposited on an area of the surface; and a pair of electrical terminals electrically coupled to the thin film electrical conductor at spaced apart locations for the flow of electrical current therebetween through the thin film electrical conductor. Preferably, the tubular substrate is a non-conductive material and the thin film electrical conductor is a molecularly bonded resistance film such as tin oxide. The terminals are preferably in the form of bus bars coupled to opposed edges of the thin film in order to produce series connected, parallel connected and/or series and parallel connected areas of thin film electrical conductor material on the tubular substrate.
The tubular resistance heater forming method of the present invention is comprised, briefly, of the steps of depositing an electrically conductive thin film on an area of an electrically non-conductive surface of a tubular substrate; and electrically coupling a pair of electrical terminals to said electrically conductive thin film at spaced apart positions for the flow of electrical current between the terminals through the thin film.